Electronic equipment recently used as portable equipment uses batteries as power source and has a multi-output power supply apparatus made of a plurality of power supply circuits in order to convert the voltage of the battery into desired power supply voltages for the various electronic circuits in the equipment. The battery voltage tends to be used down to a lower level in order to allow the equipment to operate longer. For example, input specifications for two size AA batteries are such that an initial input voltage is 3.4 V and a lower limit input voltage is 1.5 to 1.8 V. On the other hand, various power supply voltages are demanded. For example, for digital still cameras, 5 V is required for lens driving, and 1.2 V is required for a DSP (Digital Signal Processor). In the power supply apparatus, a step-up power supply circuit is required to generate 5 V, and a step-down power supply circuit is required to generate 1.2 V.
FIG. 5 shows the circuit configuration of a multi-output power supply apparatus made of a conventional step-up power supply circuit and a conventional step-down power supply circuit. As shown in FIG. 5, a step-up power supply circuit 10 is composed of an inductor 11 connected to an input power source 1 supplying an input voltage Vi, a main switch 12 made of an NMOS transistor connected to the other end of the inductor 11, and a diode 13 and an output capacitor 14 which rectify and smooth the voltage of the main switch 12.
The main switch 12 performs a switching operation to repeat accumulating and emitting energy in and from the inductor 11. Turning off the main switch 12 allows current to flow from the inductor 11 via the diode 13 to charge the output capacitor 14. The rate of on time in one switching period of the main switch 12 is defined as a duty ratio δ1, and a forward voltage drop in the diode 13 and the like are neglected. Then, a first output voltage Vo1 from the step-up power supply circuit 10 is expressed by:Vo1=Vi/(1−δ1).
In FIG. 5, a step-down power supply circuit 60 is composed of a main switch 61 made of a PMOS transistor connected to the input power source 1 supplying the input voltage Vi, a diode 22 connected to the other end of the main switch 61, and an inductor 23 and an output capacitor 24 which smooth the voltage of the connection between the main switch 61 and the diode 22.
The main switch 61 performs a switching operation to repeat accumulating and emitting energy in and from the inductor 23. This allows current to flow via the inductor 23 to charge the output capacitor 24. The rate of on time in one switching period of the main switch 61 is defined as a duty ratio δ2, and a forward voltage drop in the diode 22 and the like are neglected. Then, a second output voltage Vo2 from the step-down power supply circuit 60 is expressed by:Vo2=Vi×δ2.
In general, a PMOS transistor has worse properties than an NMOS transistor, for example, the PMOS transistor has a higher on voltage than the NMOS transistor, provided that both transistors have the same shape. Thus, when the input voltage Vi is low, for example, 1.5 to 1.8 V as described above, the main switch 61 in the step-down power supply circuit 60 has an increased on voltage, resulting in insufficient output supply. Thus, the main switch 61 of the step-down power supply circuit 60 may be composed of an NMOS transistor.
FIG. 6 is a diagram showing the circuit configuration of a step-down power supply circuit disclosed in Japanese Patent Laid-Open No. 7-222439. As shown in FIG. 6, a step-down power supply circuit 70 is composed of a main switch 71 made of an NMOS transistor connected to the input power source 1 supplying the input voltage Vi, the diode 22, the inductor 23, and the output capacitor 24, as well as a step-up inductor 72, a step-up switch 73 made of an NMOS transistor, the step-up inductor 72 and the step-up switch 73 being connected in series so as to connect the step-up switch 73 in parallel with the input power source 1, a diode 74 that rectifies the voltage of the connection between the step-up inductor 72 and the step-up switch 73, a gate power supply capacitor 75 connected between a cathode of the diode 74 and a cathode of the diode 22, a diode 76 connected between the input power supply 1 and the gate power supply capacitor 75, and a control section 77 that turns on and off the main switch 71 and the step-up switch 73.
When the input voltage Vi is lower than a predetermined value or the main switch 71 is on at a duty ratio of 100%, the control section 77 switches the step-up switch 73. Thus, while the step-up switch 73 is off, the step-up inductor 72 can charge the gate power supply capacitor 75 via the diode 74, providing a gate power supply that can turn on the main switch 71. The step-up inductor 72, the step-up switch 73, and the diode 74 constitute a step-up converter that provides a gate power supply.
When the input voltage Vi is greater than the predetermined value and the main switch 71 has been switched, the control section 77 stops driving the step-up switch 73. Thus, turning off the main switch 71 makes the diode 76 conductive. The gate power supply capacitor 75 is then charged with the input voltage Vi via the diode 76, providing a gate power supply that can turn on the main switch 71. This configuration of the diode 76 and the gate power supply capacitor 75 is called a boot strap circuit. As described above, the step-up converter or the boot strap circuit provides the gate power supply voltage for the high voltage power supply circuit 70, which uses the NMOS transistor for the main switch 71.
As described above, in the conventional multi-output power supply apparatus, made simply of the step-up power supply circuit and the step-down power supply circuit, when the PMOS transistor is used for the main switch in the step-down power supply circuit, a low input voltage increases the on voltage of the main switch in the step-down power supply circuit. This may disadvantageously result in insufficient output supply. To prevent this, the conventional method uses the NMOS transistor for the main switch in the step-down power supply circuit and uses the step-up converter or boot strap to generate the gate power supply used to drive the NMOS transistor as shown in FIG. 6. However, the gate power supply generated by the boot strap circuit is an input voltage. Consequently, double the input voltage is applied to a gate of the main switch in the step-down power supply circuit. Thus, disadvantageously, the breakdown voltage may be exceeded when the input voltage is high.